1. Field of the Invention
The present invention relates to a reflection optical system applicable to a projection optical system or a image optical system which is used in a camera using an image pickup device such as a CCD or image pickup tube and a film, and particularly to a reflection optical system suitable for a projection optical system projecting display information of a light valve under magnification, and a projection display apparatus using the reflection optical system.
2. Description of the Prior Art
In projection optical systems of projection display apparatuses and projection television apparatuses, refracting optical systems using an optical glass, and optical systems having a refracting optical system and a mirror in combination are widely known, and most of projection optical systems that are currently used are these types of projection optical systems.
However, if the refracting optical system is used, a chromatic aberration inevitably occurs because of characteristics of the optical glass. Particularly, an on-axis chromatic aberration for the long-focus telephoto type and a magnification chromatic aberration for the short-focus wide angle type are very serious problems.
For eliminating such aberrations, an optical system comprised only of mirrors has been proposed.
In Japanese Unexamined Patent Publication Nos. HEI 10-111458, 2001-222063, 2001-235681, and 2003-344772, and Japanese Patent No. 3043583, optical systems including three mirrors formed of curved surfaces and having a small number of mirrors is proposed.
However, the optical systems described in the above five documents described above have still many serious problems for practical use as a projection optical system, such that an enlargement side angle of view is narrow, an angle formed with an enlargement side image surface is large, a reduction side luminous flux divergent angle is small, and any specific proposal for a focus system is not made.
If the enlargement side angle of view is small, a distance between the enlargement side image surface and the optical system is increased when a magnification is to be increased, and therefore upsizing of apparatuses is incurred and limitations on placement positions become severer, which is not preferable. If the angle formed with the enlargement side image surface is large, a difference between the enlargement side image surface and the position of the optical system in the height direction increases, and therefore upsizing of apparatuses is incurred and limitations on placement positions become severer, which is not preferable.
If the reduction side luminous flux divergent angle is small, it means, so to speak, that the F-number of the optical system is large, and therefore a configuration making the F-number smaller and increasing the reduction side luminous flux divergent angle is desired. It is difficult to put the optical system to practical use unless a specific proposal is made on what focus system is employed, e.g. how to shift which member to perform focusing.